1. Technical Field
The present invention relates to a cam shaft for a fuel injection pump provided in an in-line internal combustion engine.
2. Background Art
As shown in FIG. 3 of the accompanying drawings, a fuel injection pump 1 of an in-line internal combustion engine (diesel engine) includes a plunger 2 for pressurizingly delivering fuel, a delivery valve 4 located between a fuel injection tube 3 and the plunger 2, a control sleeve 5 and a control rack 6. The control sleeve 5 and control rack 6 control, in combination, an amount of fuel injected. The plunger 2 contacts a cam 8 of a cam shaft 7 via a tappet 9 and the fuel injection pump 1 reciprocates up and down in accordance with a cam profile to inject fuel of predetermined pressure.
Incidentally, the cam profile P of the conventional cam shaft 7 has a lift increment segment 11 and a lift decrement segment 12. The lift increment segment 11 linearly reaches a maximum lift position 10 and the lift decrement segment 12 gently returns to a zero-lift position from the maximum lift position 10. Therefore, as shown in FIG. 4, the drive torque of the cam 8 has a positive area 13 (a torque indicated by "A" in the illustration is added) in which a tappet 9 is lifted during the lift increment section 11 so as to pressurizingly transfer the fuel and a negative area 15 in which a spring force from a spring 14 pressing the tappet 9 against the cam 8 acts during the lift decrement section 12. In case of a plural-cylinder engine, plungers 2 of the same number as the cylinders are provided and the cams 8 have different phases to move the plungers 2 at predetermined timings.
Incidentally, since the period of the positive and negative areas 13 and 15 of each cam 8 is short, positive and negative torques appear in a single cam shaft 7.
Because of this, as shown in FIG. 5, particularly in the case of a gear-driven diesel engine, each time the torque fluctuates between the positive and negative area, a contact face 18 of a gear 16 of the fuel injection pump side with a gear 17 of the drive side moves backward (clockwise in the drawing) by a gear lash as indicated by a double dot line, thereby raising the problem that a gear noise is produced.